We're going to bring in now our national security correspondent David Welna to help us answer some of the questions we heard there. First of all, David, help us understand how these national emergency declarations work. Kind of what do they activate, so to speak?

DAVID WELNA, BYLINE: Well, Congress has passed more than 400 statutes over the many years. There have been national emergency declarations going all the way back to George Washington that allow a president to act very swiftly in the situation of a national emergency. And the thinking is that Congress would just be too cumbersome, and so he, once he invokes a national emergency, activates those powers.

Now, as Tam said, he has to say which statutes he's actually going to be using, and many of those statutes have very clear guidelines about what he can do with the statutes and what he can't do and how much money he can use and so on. So we don't know exactly what he's going to invoke yet, but there are some clues already about what that might be.

CORNISH: Can you talk about those clues? As you said, people have been speculating in Washington about this idea for many weeks.

WELNA: Well, you know, he's going to raid money from some account that's already been dedicated to something else. And the account that's the biggest that Congress approves is of course defense spending. That's almost two-thirds of the money that Congress approves each year - is for defense. And there's money in military construction that would be sufficient to make up the difference in what he wants for the wall and what Congress is giving him.

CORNISH: And we should remind people that figure at one point was some 5.7 billion - right...

WELNA: Five-point-seven billion versus 1.375 billion, and...

CORNISH: ...Which is what he'd be getting in this compromise package.

WELNA: That's right. But of course he would be going against projects that have already been planned - rebuilding an airbase in Florida that was destroyed by a Hurricane, Camp Lejeune in North Carolina, things like that that have Republicans like Marco Rubio quite upset about seeing that money being taken for something that Congress said, we don't think you need to spend that much money on.

CORNISH: Talking more about Congress, we heard about Nancy Pelosi and Senate Minority Leader Chuck Schumer saying that this was a lawless act, a gross abuse of power. What are their options in terms of challenging this and on what grounds?

WELNA: They can pass a resolution that would have to pass both chambers with majorities, and they'd have to have Republicans joining them in the Senate to do so that would then have to be sent to President Trump that would terminate his decree of a national emergency. If he refuses to sign it, if he vetoes it - that would be his first veto as president - it would be sent back for a veto override, and they'd have to have two-thirds of each chamber to override his veto.

And I think that what Mitch McConnell was sort of saying on the Senate floor this afternoon was that, look; I'm behind the president on this; fellow Republicans, join in on this with me. And I think President Trump is betting that a two-thirds majority can't be mustered in either chamber to override the action that he is expected to take soon.

CORNISH: And people have talked about how unusual this is. Is there anything historically we can look back to?

WELNA: I don't think that there is a case where a president has gone around Congress and invoked these statutes to get money that Congress refused to give him. President Truman did try to nationalize the steel mills in 1952, but it was the Supreme Court who stopped him from doing that. This would be somewhat unprecedented.